1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly to diffusion layers for such fuel cells.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and generally requires expensive components, which occupy significant volume, reformer based systems are presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised of predominantly of methanol or methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate (more specifically, slow oxidation of the fuel mixture will limit the cathodic generation of water, and vice versa).
Typical DMFC systems include a fuel source, fluid and effluent management systems, and air management systems, as well as a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system generally take place within the MEA. In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the membrane electrolyte, which is impermeable to the electrons. The electrons travel through an external circuit, which connects the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
A typical MEA includes a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is Nafion ® a registered trademark of E.I. Dupont Nemours and Company, a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the catalyzed anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM through the load.
Diffusion layers typically are fabricated of carbon paper or carbon cloth. In some cases, a diffusion layer may include a coating made of a mixture of high surface area carbon powder and polytetrafluoroethylene (such as Teflon ®, commercially available from E.I. DuPont Nemours and Company, generically referred to herein as “PTFE”). The PTFE component has a function of wet proofing the diffusion layer, but as the cell reaction proceeds, the carbon paper or carbon cloth can become saturated with the fuel mixture, water or other liquid. If this occurs, the ability of the anode diffusion layer to adequately deliver the fuel mixture to the catalyzed membrane, and evolve carbon dioxide from the active area of the anode is diminished, and the performance of the DMFC and DMFC system is compromised. As noted, the fundamental reaction that occurs at the anode aspect of a DMFC is the anodic disassociation of the fuel mixture into carbon dioxide, protons and electrons, which electrons produce the electricity generated by the fuel cell. When the fuel mixture is introduced to the anode catalyst, (typically disposed on or in proximity to the membrane electrolyte), gaseous carbon dioxide is formed. The carbon dioxide is a byproduct of the electricity generating anode reaction, and is removed to improve efficiency of the fuel cell and fuel cell system.
However, in direct oxidation fuel cells, this gaseous carbon dioxide typically travels away from the catalyzed surface of the PCM through the diffusion layer, and ultimately into the anode chamber, which contains the liquid fuel supply. This can prevent liquid fuel from passing through the diffusion layer and from being introduced to the anode aspect of the PCM. The carbon dioxide can also form a bubble, which impedes the mass transport of the fuel mixture to the anode diffusion layer and hence the catalyzed membrane electrolyte resulting in an insufficient amount of fuel being delivered to the catalyzed membrane electrolyte. Either of these occurrences may prevent fuel from being introduced to at least a portion of the catalyzed PCM, effectively reducing the size of the catalyzed PCM, and limiting the power output of the fuel cell and the fuel cell system.
In addition, anodically generated carbon dioxide can actually displace the volume of fuel that can be held in the anode chamber. This typically occurs when a volume of carbon dioxide coalesces on the surface of the anode diffusion layer on the aspect opposite the membrane electrolyte. This volume displacement can further interfere with normal fluidic processes within the fuel cell system.
Further, as CO2 passes through the fuel supply in the anode chamber, it comes in contact with the concentrated fuel in the fuel cell and carries out this high concentration fuel solution and water vapor away from the catalyzed membrane, further reducing the cell efficiency.
Some carbon dioxide buildup can be eliminated from the system using a gas-permeable material disposed generally parallel or in close proximity to the anode diffusion layer. In this manner, some CO2 can be eliminated from the anode compartment and vented out of the system. A gas-permeable material has been described in commonly owned U.S. patent application Ser. No. 10/078,601 filed Feb. 19, 2002 for a SIMPLIFIED DIRECT OXIDATION FUEL CELL SYSTEM. Although the CO2 is removed from the anode compartment using said a gas-permeable membrane, the CO2 generated at the catalyzed membrane during the anodic reaction still travels through the tortuous path created by the structure of the diffusion layer and thus can still build up on the anode side prior to its reaching the liquid/gas phase separator, such as the gas-permeable membrane, or otherwise impede the flow of liquid fuel to the catalyzed anode surface of the membrane electrolyte. In addition, the CO2 can form weak bonds with the high concentration methanol, and/or water vapor and may be difficult to separate without using a gas/liquid separator within the fuel cell. Other schemes and designs to facilitate the delivery of fuel to, and eliminate anodically generated effluent gasses from the fuel cell system typically employ pumps or other active fuel delivery or byproduct removal mechanisms to manage delivery of the fuel mixture to the catalyzed membrane, as well as to manage the removal of carbon dioxide. These mechanisms, however, can increase the complexity of the fuel cell system and can give rise to issues regarding orientation independence of the entire fuel cell system. More specifically, when such mass transport or active management components are needed, it may be preferable to maintain the fuel cell system in a certain orientation (relative to vertical) to more effectively deliver fuel to the catalyzed surface of the PCM, which may make application of a DMFC system difficult for certain mobile applications.
There remains a need therefore for a diffusion layer that eliminates a substantial portion of the anodically generated carbon dioxide (or other gaseous effluent) and which prevents the accumulation of said carbon dioxide (or other gaseous effluent) within the anode chamber of the fuel cell. There remains yet a further need for a means by which carbon dioxide can be removed from a fuel cell system, which does not increase the complexity of the fuel cell system and provides some measure of orientation independence.
It is thus an object of the invention to provide a diffusion layer that removes the CO2 generated on the anode face of the catalyzed membrane, while effectively delivering fuel to the catalyzed membrane, and removing carbon dioxide from the anode side of the fuel cell prior to its reaching and passing through the anode diffusion layer.